1. Field of the Invention
This invention relates to a second-order nonlinear optical material which has a second-order, nonlinear optical effect and exhibits little birefringence along the direction of transmission of a beam, and a method for making the same. The invention also relates to optical modulation devices, which comprise the nonlinear optical material as an electrooptic element and which are useful for measurement of the variation in electric field (voltage) or as an optical switch for telecommunication and also as a phase or other modulator, temperature, and the like.
2. Description of the Prior Art
Known second-order nonlinear optical materials used as an optical modulation element include, for example, optical crystals of LiNbO.sub.3 (hereinafter referred to simply as LN), Bi.sub.12 SiO.sub.20 (hereinafter abbreviated to BSO), Bi.sub.12 GeO.sub.20 (hereinafter abbreviated to BGO), Bi.sub.4 Ge.sub.3 O.sub.12, and the like. According to "Optical Fiber Sensors" (published by Ohm Co., Ltd. and edited by Takayosi Ohkoshi (1986), pp. 149 to 153), optical modulation devices using these nonlinear optical materials have been developed for optical communication systems and also as an optical fiber sensor for measuring high voltage.
In recent years, in order to reduce the number of optical elements used in optical fiber sensors, studies have been made on optical fiber sensors of the type wherein lenses and a mirror are omitted from the sensor, and instead, an magnetooptic element or an electrooptic element is assembled in the light path of an optical fiber. This type of sensor is described, for example, in Japanese Laid-open Patent Application Nos. 5-297086, 6-74979, and 8-219825. Quite recently, it has been found that when an optical fiber is poled, a second-order nonlinear optical effect is produced. Optical modulation devices using the poled optical fiber have now been made as described, for example, by A. C. Liu et al in Opt. Lett. Vol. 19, pp. 466-468 (1994), by T. Fujiwara et al in IEEE Photonics Lett. Vol. 7, pp. 1177 to 1179 (1995), and in Japanese Laid-open Patent Application No. 9-230293.
However, with optical modulation devices including an optical fiber sensor, in which Ln having a large spontaneous birefringence is used, for example, it is necessary that an input beam be controlled so as to make an angle of axial deviation at around 0.1 to 0.2 or below. This is because if the incident angle of the beam axially deviates, the following two problems arise,
(1) The spontaneous birefringence caused by deviating an incident beam from the principal axis of a crystal becomes greater than a birefringence caused by the electrooptic effect, with the result that the degree of modulation changes greatly from a predetermined value.
(2) Because of the temperature dependence of spontaneous birefringence and nonlinear optical constant (electrooptic constant), the degree of modulation depends greatly on the temperature characteristic.
In order to solve these problems, it may occur to use crystals which are substantially free of any 'spontaneous birefringence. Known nonlinear optical materials or crystals, which do not exhibit any spontaneous birefringence, include BGO, BSO, Bi.sub.4 Ge.sub.3 O.sub.12 and the like. However, both BGO and BSO, respectively, have rotary optical power (i.e. the effect of the plane of polarization being rotated in proportion of the length of the crystal), so that the crystal length cannot be made large, with the attendant problem that the degree of modulation of a beam cannot be optionally set and the degree of modulation cannot be sufficiently increased as described in the above-mentioned "Optical Fiber Sensors", edited by T. Ohkoshi. On the other hand, Bi.sub.4 Ge.sub.3 O.sub.12 undesirably involves a DC drift at high temperatures, thus presenting the problem that when used as an optical modulator, this material does not ensure a stable temperature characteristic. This is particularly set out, for example, by O. Kamada (Appl. Phys. Vol. 32 (1993), pp. 4288 to 4291).
In an optical fiber sensor of the type wherein an ordinary electrooptic element is set in position in an optical fiber, no lens is used. Where LN, which has a small tolerable range with respect to the angle of axial deviation, is used as an electrooptic element, there arises the problem that the performance of the resultant device undesirably depends greatly on the temperature. Alternatively, if liquid crystals are used, problems are involved in that the response speed becomes very low, an abrupt change of voltage cannot be measured accurately, and the liquid crystal may be solidified when used at low temperatures.
Where part of an optical fiber is poled and used as an electrooptic element, a problem as experienced in the case of a sensor wherein LN crystal is used as an electrooptic element and an incident beam is deviated from an optical axis (Z axis). More particularly, if an optical fiber is poled at part thereof, not only the nonlinear optical effect (electrooptic effect), but also the anisotropy of refractive index (spontaneous birefringence) develops. When such a poled fiber is used in an optical fiber sensor, it is difficult to obtain an optical fiber sensor with intended characteristic properties. This problem does not occur in known optical modulators wherein a change in refractive index based on the electrooptic effect of one of principal dielectric axes is utilized, and in fact, has not been recognized.
In an optical modulator proposed, for example, in Japanese Laid-open Patent Application No. 9-230293, the electrooptic effect alone is taken into account, and no mention is made of any optical device utilizing spontaneous birefringence. Accordingly, the resultant modulator has poor linearity. In this instance, two holes are made in the clad portion of an optical fiber so as to insert electrodes. As a result, there is produced spontaneous birefringence which is ascribed to the anisotropy of the sectional structure of the optical fiber and which is much greater than the spontaneous birefringence produced according to a poling treatment. This optical fiber has such a function as a so-called "polarization preserving fiber", and the state of polarization of a beam inputted from portions other than principal dielectric axes (i.e. a line connecting two pairs of holes and a direction vertical to the line) becomes very unstable. If such an optical fiber is under varying temperature conditions or is applied with an external pressure thereto, the state of polarization of the beam changes considerably. When this optical fiber is used as an electrooptic element, and a beam, which has the direction of polarization different from the principal dielectric axes, is inputted to the fiber, the degree of modulation greatly changes by changing a temperature, for example, only by several degrees in centigrade. Thus, the electrooptic element has a very poor temperature characteristic and a large distortion rate.